1. Field of the Invention
The invention relates to a door intercom and in particular a door intercom having a door station and at least one inside station connected therewith, and having a speech function with the functions "calling inside station from door station", "talking between door station and inside station", and "opening door from inside station".
The invention relates to a door intercom designed as a hands-free system. In other words the user speaks directly into the instrument not only at the door station but also at every inside station, without using a handset.
2. Description of the Related Art
Known door intercoms with these functions, which operate as hands-free systems, are designed either as one-way intercom systems or as two-way intercom systems. In one-way intercom systems, the speaking direction is changed by manually switching the speaking direction at the inside station. One-way intercom systems of this kind are also known as two-wire systems, in other words a two-wire connection is sufficient between the door station and the inside station. In known two-way systems, a connecting line consisting of at least three conductors is necessary for the connection between the door station and the inside station.
In retrofitting buildings with door intercoms which previously had only simple bell systems, it is desirable of course to install the system by reusing the existing two-wire bell lines, since expanding the bell wire installation by an additional wire would entail an unacceptable expense (at least) if the bell wires were not laid in a pipe but without a pipe and in the plaster, as is usually the case in old buildings.
For this reason, only two-wire door intercoms and not door intercoms that operate with more-than-two-wire connections can be added easily to old buildings with conventional bell systems.
Although two-wire two-way intercoms are already prior art as door intercoms, they are not made as hands-free installations; instead they have inside stations with handsets, i.e., telephones. These known systems are described in German Patent 21 56 517 and 25 47 88.
In these known door intercoms designed as telephone systems, good speech transmission at the inside station is guaranteed since the incoming sound is supplied directly to the user's ear by means of the handset and its loudspeaker, and the outgoing sound is received directly at the user's mouth by means of the microphone on the handset, but there are the disadvantages, frequently viewed as significant, that:
the conversation with the visitor cannot be conducted in a hands-free manner, because one hand has to hold the handset and the other hand must be used to operate the door-opening button or the like; PA1 the inside station with the handset and its flexible connection, which must necessarily be of a certain length, is relatively bulky and is exposed to considerable risk of damage, firstly because these devices are often not treated carefully by subscribers, and secondly because the unit which projects for a distance from the wall is exposed to the risk of impact from persons going by.
However, a two-way system of this kind that uses a handset cannot simply be produced by mounting a fixed loudspeaker and a fixed microphone as a hands-free unit in a housing. Because of the greater distance between the ear and the mouth and the respective sound converting devices, particular transmission problems arise, higher amplifications of the speech currents are required, and, in contrast to the handset, there is a much greater acoustic coupling between the sound converters, in other words between the loudspeaker and the microphone. This means however that the additional feedback produced by two-wire operation between the transmitting and receiving directions must be kept very low in order to achieve the required protection against feedback howling with sufficiently high amplification in both directions, so that during normal speech the sound pressure is still sufficiently high at the other end of the transmission path.
In addition, the inside station, which has no power line connection of its own, must be supplied through the two-wire connecting line to the door station with sufficient DC power to operate the required amplifier, especially the loudspeaker amplifier, and sufficient conversation currents must be carried for reliable transmission. In addition, the terminal resistance of the inside stations, which differs according to the length of the line and the cross section of the line, makes clear balancing difficult both in the door station and in the inside stations. As a result, the total amplification in the two transmission directions, which is otherwise possible with a given protection against howling is reduced in both transmission directions with the result that the loudness at one or both locations is too low.
Because of the above problems, there are presently no two-wire door intercoms designed as hands-free units for two-way operation.